1. Field of the Invention
The present invention relates to image processing, and more particularly, to image processing for use in reproducing an image from an original image with a frequency component or components emphasized which are higher than a specific spatial frequency.
2. Description of the Prior Art
In image processing, especially in a digital data processing, a so-called unsharp masking process may often be involved in which a spatial frequency component or components of a specific frequency band included in a digitalized video signal are enhanced so as to produce an image that is suitable for a specific usage. In a medical diagnosis, for example, there is a kind of radiation photography system using a stimulable phosphor sheet. In such a system, a radiation sensitive sheet, made of stimulable phosphor, is exposed to an imagewise radiation to store an original image therein, which is then read out by illumination of a stimulating ray to be recorded as a visible hard copy of image on a recording medium.
A latent image of an object under inspection, such as of a living body, which is temporarily stored in a sheet of stimulable phosphor as energy distributions of radiation, is scanned with a stimulative light beam to cause secondary emission of light from the sheet in dependence upon the stored latent image. The secondary emission of light from the phosphor sheet is in turn converted into an electrical signal indicative of the image. The electrical signal is converted into a digital form, which is then processed so as to cause a specific frequency component or components to be emphasized, effecting enhancement in sharpness of edge portions of the image. In a medical diagnosis, for example, such unsharp masking may advantageously be used to produce an easily readable picture of blood vessels or foci, for example, with a fine gray scale tone emphasized.
More specifically, according to Japanese Patent Laid-Open Publication No. 163472/1980, for instance, a video signal S' which has been processed under unsharp masking is represented by the following expression: EQU S'=S.sub.org +.beta.(S.sub.org -S.sub.us), (1)
where S.sub.us is an unsharp masking signal associated with a very low spatial frequency, or infralow frequency, which is obtained at sampling points, or pixels, in scanning lines formed by scanning a stimulable phosphor sheet having a latent image stored therein with a stimulative ray in raster fashion, S.sub.org is an original video signal readout from the stimulable phosphor sheet, and 8 is an emphasizing coefficient.
It is to be noted that the unsharp masking signal S.sub.us is available from scanning points on an unsharp image, or unsharp masking, into which the original image has been processed so as to only include frequency components lower than such an infralow frequency component. In addition, the words "original video signal" may also include a signal which is resultant from an image processing which is common in the art, such as nonlinear (e.g. logarithmic) amplification for nonlinearity compensation or band suppression. Also, the emphasizing coefficient .beta. may be a constant, and in general may also be a function .beta.(S.sub.org, S.sub.us) of the original video signal S.sub.org and the unsharp masking signal S.sub.us.
According to Japanese Patent Laid-Open No. 75139/1981, when a latent image stored in a stimulable phosphor sheet is scanned in raster fashion, the unsharp masking signal S.sub.us may be derived from averaging by simple addition the original signals S.sub.org at the scanning or sampling points in a rectangular region formed by two straight lengths of line, parallel to scanning lines, and two straight lengths of line, perpendicular to the scanning lines. Assuming that, for example, an area or region of an original image is scanned which includes N scanning lines with each scanning line including M sampling points or pixels, then the unsharp masking signal S.sub.us is got from the following expression: ##EQU1## Combining the expressions (1) with (2), unsharp masking process depends upon the following expression: ##EQU2## Generally, .beta. may be a function of S.sub.org and S.sub.us, and the expression (3) may in general be expressed by the following formula: ##EQU3## where .gamma. is a nonlinear function for use in a known gradiation conversion.
Specifically, in the case of the number M of sampling points in a scanning line equal to the number N of the scanning lines in the rectangular region discussed above, the expression (4) will be transformed into ##EQU4##
The operations in accordance with expressions (3), (4) and (5) are substantially not different from an ideal masking operation with respect to improvement in medical diagnostic performance of radiation images, and in addition include an averaging by simple addition, resulting in shortening an operational period of time as well as simplifying the system configuration for image processing.
In the operations in accordance with formulae (3), (4) and (5), it takes as much time for operations of summing the original video signals with respect to the entire pixels involved in a rectangular area, namely, ##EQU5## as for that of the remaining portions of the formulae (3), (4) and (5).
As depicted in FIG. 3, for example, a square mask 500 is formed by four sides of equal length including N pixels each, with horizontal scanning made in the direction of the arrow H and vertical scanning in the direction of the arrow V. A total optical density C.sub.i resultant from summation of an optical density S.sub.i,j of a pixel (i,j) in the direction of the arrow V is represented by the formula: ##EQU6## where the pixel at the center of mask 500 is represented by (a,b) in the coordinates. The total density T with respect to the entire mask 500 is then given by ##EQU7##
During the scanning of the n-th line, the scanning beam reaches pixel (m,n), so that the unsharp masking operation is shifted from S.sub.m-1,n to S.sub.m,n. The operation is performed by reading the optical density S.sub.m,n to add it to the sum density resultant from totalizing or accumulating the densities on the preceding pixels to the pixel (m,n) on the m-th column, with subtraction from the sum density made of the optical density of pixel (m,n-N) on the m-th column, which pixel has gone out of mask 500 due to the shift of operation. Namely, this operation is indicated by the following expression: EQU C.sub.m .rarw.C.sub.m +S.sub.m,n -S.sub.m,n-N
Successively, from the total density resultant from the preceding masking operation, subtracted is the sum density C.sub.m-N of the pixels in the (m-N)th column which has not been included in mask 500, with the sum density C.sub.m of the pixels in the m-th column added which is newly involved therein. This is represented by the following expression: EQU T.rarw.T+C.sub.m -C.sub.m-N
Using T thus obtained and original video signal S.sub.org, unsharp masking operations will be performed on mask 500.
Following table I shows an example of operational periods of time during which addition operations with respect to an unsharp masking were executed by a bit-slice type microprocessor in which it takes one microsecond for reading/writing and addressing a storage location, 0.3 microsecond for addition and subtraction each, 0.9 microsecond for multiplication, and 3 microsecond for division. In Table II, operational periods of time for emphasis and gradiation processing with respect to .beta. and .gamma. are also shown. Additionally, the microprocessor used had a storage including a table which defines an emphasizing function .beta. and a nonlinear function for gradiation conversion .gamma.. It should be noted that the period of two microseconds required for transferring data was specific for the system in accordance with the present invention.
As clear from those tables, it takes comparable periods of time for executing the additions with respect to the unsharp masking and for processing emphasis and gradiation. To the respective operations, a small period of time is inpractice added which is necessary for error control, periphery processing of a frame, and timing control between the image reader and the image recorder. In any case, however, it took 25-30 microseconds for image processing per unit pixel.
TABLE I ______________________________________ Read S.sub.m,n 1 microsecond Write S.sub.m,n 1 Read C.sub.m 1 Read S.sub.m,n-N 1 C.sub.m .rarw. C.sub.m - S.sub.m,n-N .3 C.sub.m .rarw. C.sub.m + S.sub.m,n .3 Write C.sub.m 1 Read C.sub.m-N 1 T .rarw. T - C.sub.m-N .3 T .rarw. T + C.sub.m .3 Read S.sub.org 1 Transfer S.sub.org and sum 2 Total 10.2 microseconds ______________________________________
TABLE II ______________________________________ Transfer(Read) S.sub.org, sum 2 microseconds Division .alpha. .rarw. sum/N.sup.2 3 .DELTA. .rarw. S.sub.org - .alpha. .3 Overflow and underflow control 2 during calculation steps Read .beta.(S.sub.org, S.sub.us) 1 Multiply .beta.(S.sub.org, S.sub.us) with .DELTA. .9 S.sub.org + .beta.(S.sub.org, S.sub.us) .times. .DELTA. .3 Read .gamma. table 1 Write memory 1 Total 11.5 microseconds ______________________________________
In a radiation photography system using stimulable phosphor sheets, or image storage panels containing stimulable phosphor, on which images have been stored at radiation exposure sites or stations are collected to an image processing station or center, in which information on the stored images are read out from the thus collected storage panels to be data processed for reproduction as easily recognizable images. It is required for the image processing systems provided in the center to deal with a huge amount of image data for a very short period of processing time. It is therefore advantageous for those image processing systems to execute operations on such unsharp masking during as short period of time as possible.
It is therefore an object of the invention to provide an image processing system which is capable of processing unsharp masking on a real-time basis in a shortened period of operational time with respect to a large amount of image data.